Effect Of PH And Humic Acid On Heavy Metals Releasing From Containmented River Sediment

Heavy metal pollution is one of the most serious environmental problems in the world;pH and humic acid(HA)are two major factors affecting the release of heavy metals in sediments.In this paper,firstly,heavy metals including Cd,Ni and Cu release from river sediment were investigated by a leaching test according to pH changes.The test was performed in the range of pH values of 0-14 using deionised water and river water as leaching solutions and Experimental results are modeled with the Visual MINTEQ geochemical software to predict metal release from sediment using both leaching liquids.Secondly,using Tessier sequential extraction method,five kinds of different speciation of heavy metals in sediment was extracted and the results can be used to study the mechanism of heavy metal release from sediment.Then humic acid(HA)was extracted from the sediment.The thermodynamics and kinetics of the adsorption of Cr(VI)and Cd(II)onto solid soil-derived HA have been investigated at optimum conditions of pH(5.5 ± 0.1),metal concentration(10–100 mmol l-1)and different temperatures(293–323 K).The suitability of adsorption models such as Freundlich and Langmuir to equilibrium data was investigated.Leaching test results showed that the release amounts of Cd,Ni and Cu are in the range of 1.0-2.7 mg/L,8.0-14 mg/L,and 0.6-3 mg/L respectively with the leaching solution of deionised water.The maximum release of Cd in the sediments is reached at pH0,and the minimum release of Cd in the sediments is got at pH values of 5 – 9.The maximum leaching for Ni in the sediments is observed at pH0 – p H4.Ni release decreases to minimum values as the pH increases from pH5 to pH 10.Ni increases again with the p H arise from pH10.As the pH increases from p H0 to p H6,Cu release in the sediments decreases then comes to be stable at pH6 – pH10.As far as the river water was used as the leaching solution in the test,the results show a similar metal leaching concentrations and tendencies to that of the deionised water as leaching solution.The results of Tessier sequential extraction indicates that Cd of the residual state has a obvious precipitation under the acidic condition,especially in the range of pH0-pH4 with the residual of Cd over 50%.The exchangeable content of Ni decreases with the increase of pH value in the range of 0–5.The speciation of Cu in the sediments changes significantly between pH0 – pH9,especially for the state of bound to carbonates.The distribution and speciation of heavy metals in the sediments after leaching test are related to the competitive relation between H+,metal ion and the adsorption of the colloid in the sediment.Base on the effect of p H on the leaching of Cd,Ni and Cu from polluted river sediment in the tests,more accurate information could be got to assess the risk related to metal release from sediments once it is exposed to the changed acid/ alkali water conditions.The thermodynamic and kinetic results of adsorption of Cr(VI)and Cd(II)by HA are known that the adsorption was well described by Langmuir isotherm model in multi-detectable steps.Adsorption sites,i(i = A,B,C)with different capacities.The adsorption is endothermic,spontaneous and favorable.Adsorption sites A and B are selectively occupied by Cr(VI)cations while sites A and C are selectively occupied by Cd(II)cations.Different kinetic models are applied and the adsorption of these heavy metals onto HA follows pseudo –second-order kinetics and equilibrium is achieved within 24 h.The adsorption reaction is controlled by diffusion processes and the type of the adsorption is physical.